Southern Spiders #1

Matthew Sarver — Wed, 19 Aug 2009 03:56:06 +0000

I don’t know about you, but I love spiders. While here in South Carolina, I was lucky to find and photograph several common and attractive species. This is the first installment in a series of posts in which I’ll highlight these species for the enjoyment of all of you arachnophiles out there!

Lined Orbweaver, Mangora gibberosa
Family: Araneidae

The Lined Orbweaver is a common spider that is widespread throughout the eastern United States. In the southeast, it is especially abundant in grassy pond pine (Pinus serotina) savannas like the one shown below.

Strung among the grasses throughout the savanna are small, white, silken rings that are conspicuously evident against the sea of yellowish-green vegetation. Upon closer examination, the white ring is at the center of a larger orb-web, the rest of which is invisible from a distance. The white part of the web is called the stabilimentum, a structure found in the webs of many species of orb-weavers (Araneidae).

The purpose of the stabilimentum has been debated among arachnologists for a long time. Hypotheses range from prey attraction to camoflauge for protection from predators to warning signals that keep megafauna from accidentally destroying webs. Phylogenetic work suggests that web-decoration with stabilimenta evolved several different times in many araneid lineages (Scharff & Coddington 1997).

My personal observations of the Lined Orb-weaver indicate that, in this species, one use of the stabilimentum is as a refuge from potential predators. When the web is approached closely or bumped, the spider quickly runs to the opposite side of the web from the intruder and hides behind the stabilimentum, which is sized perfectly to conceal the body of the spider.

How well this works, however, is open to debate. Bruce et al. (2005) tested the visibility of stabilmentum silk to birds and bees. They found that in one of their study species, the chromatic contrast between the spider and the silk was significant enough that the stabilmentum probably provided inefficient camoflauge from avian predators at close range. It’s also possible that, since they are visible to bees and other insects, the small, discoid stabilimenta of the Lined Orb-weaver may attract prey to the web by mimicking flowers.

Whatever their adaptive significance, the sight of dozens of these small, silken rings dotting the vegetation is a fantastic experience for the naturalist willing to look closely. The spider responsible for the web is pretty cool-looking as well!

For a nice review of the literature on this topic, check out Matt Bruce’s web site on web decorations.

Citations:

Bruce, M.J., Heiling, A.M., Herberstein, M.E. 2005. Spider signals: are web decorations visible to birds and bees? Biology Letters 1: 299-302.

Scharff N, Coddington J.A. 1997. A phylogenetic analysis of the orb-weaving spider family Araneidae (Arachnida, Araneae). Zool. J. Linn. Soc. 120: 355–424.

A Helpful House Sparrow

Matthew Sarver — Fri, 03 Jul 2009 23:57:14 +0000

The House Sparrow (Passer domesticus) is one of those critters that birders love to hate. The species makes a habit of usurping the nest sites of native species, especially bluebirds, and is therefore reviled by many a birder.

Initially released on this continent in the early 1850s in New York City and parts of New England, followed by many more releases across the U.S. and Canada during subsequent decades, the Old World sparrow rapidly established outposts on New World soils. By the 1880s, the species was well-established in many cities, and its impacts on native avifauna were becoming obvious. The bird that had once captivated the countless Americans who aided in its rapid spread had already worn out its welcome.

When watching these birds, it’s not difficult to see why they have persisted for the past century. Their ability to thrive in urban settings, scavenging discarded french fries and other bits of refuse, is notorious. Anyone who has watched House Sparrows making themselves at home inside warehouse stores or building sloppy nests of dry grass in every conceivable type of lamppost can attest to their adaptability. I have to admit that I’m ambivalent about the species. The scientist in me dislikes the impact of the House Sparrow on native birds, but the humanist in me loves the plucky spirit of these little tank-like city-dwellers.

Yesterday I witnessed an event that made me smile. As I was about to get out of my car, I noticed a female House Sparrow that had flown to the ground nearby with something shiny and green in her bill. I froze in the driver’s seat, window down, and watched. Not ten feet away, the bird lit on some reddish landscaping gravel. At this range I could tell what the mysterious object was: not one, but two Japanese Beetles! The unfortunate beetles were attached to each other for the purposes of mating (a position referred to by entomologists as in copulo) and had no hope of disuniting in time for either to escape! The sparrow settled down to lunch and quickly munched most of the first beetle of the pair, while the beetle’s mate could only wriggle in vain, still attached to the little that remained of its mate. After briefly dropping the remaining beetle into the stones, the House Sparrow deftly picked up a piece of gravel, dropped it aside, and reclaimed its prey.

Two Japanese Beetles down, millions more to go. One introduced pest species feasting on another. Unfortunately, the energy-rich beetle innards, recycled, no doubt, from some gardener’s prized roses, were going to help produce more House Sparrows.

Such is the way of the urban jungle. At least now I feel a little better about my guilty admiration for Passer domesticus.

House Sparrow (Passer domesticus) by J. Garg. Distributed under the terms of a Creative Commons Attribution ShareAlike 3.0 License (http://creativecommons.org/licenses/by-sa/3.0/)

Matthew Sarver » Predator

Southern Spiders #1

A Helpful House Sparrow